Power steering with hydraulic power assistance

ABSTRACT

In connection with a servo-assisted steering system with hydraulic servo-assistance for motor vehicles, a steering shaft ( 15 ), which is provided with a steering wheel ( 16 ), is connected via a torsion spring ( 14 ) with an input member of a steering gear ( 2 ). An output member of the steering gear ( 2 ) is connected with the wheels ( 11 ) of the vehicle which are to be steered. In case of a relative rotation of the steering shaft ( 15 ) in respect to the input member, two work chambers ( 12, 13 ) of a servo motor of a force amplification device can be pressurized by a servo pump via a steering valve with a pressure medium. Two sensors ( 23, 24 ) for detecting a rotation angle and/or a torque are arranged on the steering shaft ( 15 ), or respectively on the output member of the steering gear ( 2 ). After the steering wheel ( 16 ) is released following a steering process, the wheels to be steered ( 11 ) and the steering gear ( 2 ) can be returned with hydraulic assistance. The amount and the direction of the hydraulic servo-assistance, as well as the return of the wheels ( 11 ) to be steered, are controlled by means of a common electronically-controlled electromagnetic ( 18 ) valve.

BACKGROUND OF THE INVENTION

The invention relates to a servo-assisted steering system with hydraulicservo-assistance for motor vehicles. Here, a steering shaft, which isprovided with a steering wheel, is connected via a torsion spring withan input member of a steering gear. An output member of the steeringgear is connected with the wheels of the vehicle which are to besteered. In case of a relative rotation of the steering shaft in respectto the input member, two work chambers of a servo motor of a forceamplification device can be pressurized by a servo pump via a steeringvalve with a pressure medium. At least one sensor for detecting arotation angle and/or a torque is arranged on the steering shaft. Asensor for detecting a steering angle is arranged on the output memberof the steering gear. This sensor can also be integrated into therotation angle sensing mechanism of the steering shaft. A returningdevice is used for the hydraulic assistance of the returning of thesteering gear and the wheels to be steered after the steering wheel isreleased following a steering process.

Such a servo-assisted steering system is known from EP 0 440 63 8 B1. Inthis case an electric motor used to affect the reaction force, which canbe noted on the steering wheel, as a function of various parameters, isarranged in addition to a complete servo-assisted steering system withhydraulic servo-assistance. Moreover, the electric motor is used tocenter the steering valve exactly in its center. Finally, it is possibleto achieve a hydraulic steering axle return in that, following therelease of the steering wheel, the steering valve is displaced past itsneutral position into a steering position, which corresponds to areversal of the steering direction. A steering system is known from U.S.Pat. No. 4,745,985, which contains several features of the invention.With this steering system the amount and the direction of the hydraulicservo-assistance, as well as the return of the wheels to be steered, arecontrolled by means of a common electronically-controlledelectromagnetic valve. Steering errors could appear in case of thepossible appearance of a malfunction in the electronic control device,which could lead to endangerment.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is based on realizing the positiveproperties present in the known servo-assisted steering system, such asthe exact centering of the valve and the hydraulic steering axle return,by simpler means without the use of an electric motor. Moreover, thesteering ability of the vehicle is to be assured if a malfunction in theelectronic system occurs.

This object is attained with the servo-assisted steering system featuredin that the amount and the direction of the hydraulic servo-assistance,as well as the return of the wheels to be steered are controlled bymeans of a common electronically-controlled electromagnetic valve whichthus acts as a return device. The electromagnetic valve can beintegrated into the actual servo-assisted steering system, or it can bearranged outside of it at any arbitrary place. The auxiliary power forthe hydraulic servo-assistance is exclusively provided by the servopump. No electric motor is required.

In order to prevent danger in case of a possibly occurring error, abypass valve is arranged between the steering shaft and the input memberof the steering gear. With other known servo-assisted steering systemsthe steering valve is located at this location. The bypass valve isdesigned as a rotary slide valve with a closed center, so that in itsneutral position there is no possible connection between the two workchambers of the servo motor. Only when an error occurs is the bypassvalve switched into its open position, so that the two work chambers ofthe servo motor are connected with each other. The two work chambers arethen bypassed, so that no pressure buildup can take place in the servomotor. In this case the steering energy is exclusively provided by themechanical actuation of the steering wheel.

Advantageous and useful embodiments of the invention are recitedsubsequently.

The electromagnetic valve can be controlled in a simple manner as afunction of at least one parameter, for example the road speed of thevehicle, the steering speed, the yaw speed, the load of the vehicle, theacceleration of the vehicle and others.

By means of an appropriate programming of the electronic unit andappropriate sensor devices it is possible to control the electromagneticvalve by means of signals, which provide information regarding thepresent position of the vehicle and regarding desired future positions.With this, the option for automatic steering is contained in theservo-assisted steering system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail in what follows bymeans of two exemplary embodiments represented schematically in FIGS. 1and 2.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained by means of an example of a ball and nutservo-assisted steering system. However, the invention can be appliedwith the same effect to a rack and pinion servo-assisted steeringsystem.

A steering gear 2 in a steering housing 1 contains a work piston 3,which is drivingly connected via a series of balls 4 with a threadedspindle 5, and via teeth 6 with a sector shaft 7. Here, the threadedspindle 5 constitutes an input member of the steering gear 2, and thesector shaft 7 an output member of the steering gear 2.

There is a connection from the sector shaft 7 via a drop arm 8 andsteering linkage 10 with at least two wheels 11 of the motor vehiclewhich are to be steered.

The housing 1 forms two cylindrical work chambers 12 and 13, which areseparated from each other by the work piston 3.

The threaded spindle 5 as the input member of the steering gear 2 isconnected with a steering shaft 15 having a steering wheel 16, by meansof a torsion spring 14. The steering shaft 15 can be divided by a crossjoint 17.

By means of an electromagnetic valve 18, for example a proportionalvalve, the two work chambers 12 and 13 can be pressurized with apressure medium, which a servo pump 19 conveys from a reservoir 20. Theservo pump 19 is driven, for example, by a vehicle engine 21, or anelectric motor.

The electromagnetic valve 18 can be designed with an open or a closedcenter. In case of a design with a closed center, a pressure mediumreservoir, not represented, is provided in addition to the servo pump19.

The electromagnetic valve 18 is regulated by means of an electronic unit22. The electronic unit 22 receives signals from at least one firstsensor 23, arranged on the steering shaft 15, for detecting a rotationangle and/or a torque. A second sensor 24 for detecting a steering angleis arranged on the output member, the sector shaft 7, of the steeringgear 2 and provides the appropriate signals to the electronic unit 22.The sensor can also be arranged with the same effect on the input memberof the steering gear 2. Further signals, which relay parameters of therespective state of the vehicle, are provided to the electronic unit 22,for example from an electronic speedometer 25 and a yaw speed sensor 26.

In the exemplary embodiment in accordance with FIG. 1, a bypass valve 27has been inserted between the steering shaft 15 and the input member ofthe steering gear 2, i.e. the threaded spindle 5. The bypass valve 27 isembodied as a rotary slide valve with a closed center. This issymbolically indicated in FIG. 1. In case of a relative rotation betweenthe steering shaft 15 and the threaded spindle 5, during which thetorsion spring 14 is turned as a function of the steering momentapplied, the bypass valve 27 can open at an actuating force of slightlyless than 450 N. In accordance with an EEU Guideline, such an actuatingforce is permissible when an error occurs in a servo-assisted steeringsystem. This bypass valve 27 is mechanically actuated, similar to arotary slide valve of a “normal” servo-assisted steering system, and istherefore very dependable. The work chambers 12 and 13 are connectedwith each other by the opening of the bypass valve 27, so that nopressure buildup can take place in the work chambers 12 and 13 of thesteering gear 2. In this case the steering gear is exclusively operatedby the manual force at the steering wheel.

The servo-assisted steering system can be laid out for automaticsteering by means of the appropriate programming of the electronic unit22. It is possible to transmit signals from a guide device, notrepresented, arranged outside of the vehicle, to the electronic unit 22.The guide device can be equipped with an induction cable in the road,for example. A video orientation or a satellite control is alsopossible. The signals provide information regarding the present positionand regarding desired future positions. The command value/actual valuedeviation is then detected by the electronic unit 22 and appropriatelycorrected steering commands are then passed on to the electromagneticvalve 18. The driver can turn off the automatic steering by action onthe steering wheel 16.

With the second exemplary embodiment in accordance with FIG. 2, thebypass valve is integrated into the electromagnetic valve 18. In thiscase, the bypass valve can be electronically controlled together withthe electromagnetic valve 18.

In this exemplary embodiment, the first sensor is embodied as anglesensor 28. A further angle sensor 29 is installed at the lower end ofthe steering housing 1, coupled with the threaded spindle 5. By means ofthis further angle sensor 29 it is possible to improve the angularresolution of the second sensor 24 arranged on the sector shaft 7.

The remaining components of the second exemplary embodiment correspondto those in the first exemplary embodiment, and are identified by thesame reference numerals.

A steering movement by means of the steering wheel 16 is detected by theangle sensor 28 at the steering shaft 15. The steering movement ispassed on from the steering shaft 15 via the torsion spring 14, which inthe process is turned as a function of the actuation moment. On itsother side the torsion spring 14 is firmly connected with the threadedspindle 5.

Turning of the torsion spring 14 causes a change of the relation of theupper angle sensor 28 in respect to the second sensor 24 on the sectorshaft 7. This relative angular change is evaluated by the electronicunit 22. The electronic unit 22 then guides an oil flow with the correctrotation from the pump 19 via the electromagnetic valve 18 to theappropriate work chamber 12, or respectively 13. Since the second sensor24 on the sector shaft 7 has a total angular deflection of onlyapproximately 90 degree, and therefore the resolution of the sensor isrelatively imprecise, it would be necessary to install a step-up gearupstream of it. The angle sensor 29, which rotates together with thethreaded spindle 5, takes the place of such an expensive step-up gear.

The two angle sensors 28 and 29, as well as the first and the secondsensors 23, 24 at the sector shaft 7, and the electronic unit 22 areredundantly designed. By means of the redundancy in the area of thesensor and the electronic unit it is possible to detect errors and todeactivate the hydraulic servo-assistance of the steering gear 2 bymeans of the electronically controllable bypass valve integrated intothe electromagnetic valve 18.

What is claimed is:
 1. A servo-assisted steering system with hydraulicservo-assistance which is used for motor vehicles, comprising: asteering shaft which is provided with a steering wheel, said steeringshaft being connected via a torsion spring with an input member of asteering gear, an output member of the steering gear which is connectedwith wheels of the vehicle which are to be steered, two work chambers ofa servo motor of a hydraulic servo-assistance device which arepressurized by a servo pump via a steering valve when there is arelative rotation of the steering shaft with respect to the inputmember, at least one first sensor for detecting one of a rotation angleor a torque which is arranged on the steering shaft, a second sensor fordetecting a steering angle which is arranged on one of the output memberor the input member, a returning device for returning the steering gearand the wheels to be steered to a predetermined position after thesteering wheel is released following a steering process, a commonelectronically-controlled electromagnetic valve which controls theamount and the direction of the hydraulic servo-assistance as well asthe return of the wheels to be steered, and a bypass valve which isarranged between the steering shaft and the input member of the steeringgear.
 2. The servo-assisted steering system as claimed in claim 1,wherein the bypass valve is a rotary slide valve with a closed center.3. The servo-assisted steering system as claimed in claim 2, wherein inan opened position of the bypass valve the two work chambers of theservo motor are connected with each other.
 4. The servo-assistedsteering system as claimed in claim 1, wherein the bypass valve is asafety valve which is mechanically actuable.
 5. The servo-assistedsteering system as claimed in claim 1, wherein the electromagnetic valveis controlled by an electronic unit as a function of at least oneparameter of the vehicle.
 6. The servo-assisted steering system asclaimed in claim 1, wherein the electromagnetic valve is controlled byan electronic unit as a function of signals which provide informationregarding the present position of the vehicle and desired futurepositions.
 7. The servo-assisted steering system as claimed in claim 6,wherein the signals are transmitted to the electronic unit of thevehicle by a guide device arranged outside of the vehicle.
 8. Theservo-assisted steering system as claimed in claim 1, wherein theelectromagnetic valve is a valve with a closed center.
 9. Theservo-assisted steering system as claimed in claim 1, wherein the bypassvalve is integrated in an electronically controllable manner into theelectromagnetic valve.
 10. The servo-assisted steering system as claimedin claim 9, wherein, in addition to the first sensor, a further thirdsensor for detecting one of a rotation angle or a torque is arranged atthe input member of the steering gear.
 11. The servo-assisted steeringsystem as claimed in claim 1, wherein the first and second sensors andthe electronic unit are made redundant.
 12. The servo-assisted steeringsystem as claimed in claim 1, wherein the electromagnetic valve is aproportional valve.
 13. The servo-assisted steering system as claimed inclaim 3, wherein the bypass valve is a safety valve which ismechanically actuable.
 14. The servo-assisted steering system as claimedin claim 13, wherein the electromagnetic valve is controlled by anelectronic unit as a function of at least one parameter of the vehicle.15. The servo-assisted steering system as claimed in claim 14, whereinthe at least one parameter is selected from steering speed, yaw speed,or a load of the vehicle.
 16. The servo-assisted steering system asclaimed in claim 14, wherein the electromagnetic valve is controlled byan electronic unit as a function of signals which provide informationregarding the present position of the vehicle and desired futurepositions.
 17. The servo-assisted steering system as claimed in claim14, wherein the electromagnetic valve is a valve with a closed center.18. The servo-assisted steering system as claimed in claim 17, whereinthe first and second sensors and the electronic unit are made redundant.19. The servo-assisted steering system as claimed in claim 18, whereinthe electromagnetic valve is a proportional valve.